1. Field of the Invention
The present invention relates generally to an aluminum assembly, such as a heat exchanger for automotive vehicles, manufactured by controlled atmosphere brazing ("CAB").
2. Description of the Related Art
It is known to provide automotive vehicles with assemblies including heat exchangers such as condensers, evaporators, heater cores and coolers generally made of aluminum or aluminum alloys. These heat exchangers are alternating rows of tubes or plates. The heat exchangers often include convoluted fins brazed to the external surfaces of the tubes and turbulators disposed within the tubes and brazed to their inner surfaces. The tubes (or fins) generally carry a clad layer which has a melting temperature lower than that of the tube (or fin) core aluminum alloy. During brazing, the clad layer melts and forms brazed joints between the tube and the fins. For example, sheets made of aluminum alloys for the core with a clad layer can be formed into shape and welded into tubes. However, if greater strength (such as pressure to burst) is required of the tube, as may be in condensers, then extruded tubes instead of welded tubes are generally used. Since it is not commercially feasible to have a clad layer on the internal surface of the extruded tube, cladding on fins is used to provide the molten material needed for brazing. It is recognized that use of a clad layer adds to manufacturing complexity, cost, and component weight.
When aluminum is exposed to air, the surface layer oxidizes and forms aluminum oxide. Thus, flux is generally an important ingredient of the brazing process. Previously, the brazing of the fins and turbulators to the tube surfaces has been carried out in a vacuum furnace. Recently, however, a process known as "controlled atmosphere brazing" (CAB) has been employed. CAB furnace brazing is preferred over vacuum furnace brazing due to improved production yields, lower furnace maintenance requirements and greater braze process robustness. The function of the flux includes displacing any oxide from the surface of the aluminum and protecting it from further oxidation, reducing filler metal surface tension, and thus promoting wetting. Chloride-based fluxes are generally not used because they are highly hygroscopic and very corrosive to the aluminum. A flux commonly used in CAB furnace brazing is Nocolok.TM. (potassium fluoaluminate represented often as "KALF"). This flux or other similarly formulated fluxes are non-corrosive to aluminum after brazing.
Magnesium is generally included in the aluminum core to improve its strength. During the high temperatures of brazing, the magnesium may migrate through the core alloy and the clad layer to the surface where it reacts with the flux, forming high melting temperature compounds, thereby rendering the flux ineffective; i.e. the flux is "poisoned" by reacting with the magnesium. This is highly detrimental to formation of a sound braze joint because they interfere with the "wetability" of the aluminum surfaces to be brazed together. To avoid this poisoning, the magnesium content in the core alloy must be below 0.1 wt. %. From the standpoint of aluminum alloy strength, however, it is very desirable to increase the magnesium content in the core alloy.
We have unexpectedly found that these problems can be overcome by providing a coating of an zinc-antimony (Zn--Sb) alloy on the aluminum component. Such a coating substitutes for the clad layer to provide the molden metal needed for the formation of brazed joints during brazing, making it feasible to use extruded tubes and unclad fins, for example, for heating exchangers. Further, the zinc-antimony coating prevents magnesium in the core from migrating to the surface and interacting with the flux which would interfere with formation of a sound braze joint, i.e. use of this coating prevents poisoning of the flux by the magnesium. We have unexpectedly found that the use of a coating of Zn--Sb on aluminum parts allows for the formation of sound, continuous joints between parts which have desirably higher levels of magnesium, even as much as 3 wt. % magnesium. The use of our zinc-antimony coating also provides brazability for the joint, corrosion protection for the aluminum core, and enhanced brazed joint strength.
In U.S. Pat. No. 4,891,275, Knoll discloses a method whereby immediately after an aluminum shape is extruded and before it is exposed to the atmosphere, it is coated with a molten metallic layer of zinc or a zinc-base alloy. This coating is to function as a fluxless soldering or low temperature brazing material when heated. Kroll teaches that the zinc alloy is preferably a zinc-aluminum alloy which may also comprise copper. The addition of aluminum it taught to improve wetting of the aluminum by molten zinc and in combination with copper results in a sufficiently ductile coating of high tensile strength in a cold state. One of the drawbacks of the '275 method is that it requires immediately coating the aluminum part after extrusion.